Paving attention

We've had the paving slabs reset in our area this week. This was the scene outside my patio this morning before the lads came by to finish off.
At least its flat now so hopefully wont go flying, its a shame there are litchens (not sure if thats the right spelling) on these ones as its that stuff I slipped on one dewey morning.
I want them gone, they suggested acid! Right where would I get acid from I ask myself.
Patio cleaner doesnt work unless you hack at them with a stanly knife first! I know I tried it.